The Shallow Sky

Planets in April

Akkana Peck

... It's a perfect time to look for details in the ring system, such as gaps in the rings, and the ever elusive "spokes" ...

Shallow sky afficianado Akkana Peck conducts a session of the observational astronomy class at Houge Park. For future class times, see the SJAA Activities Calendar on the front page.

Jupiter continues an excellent apparition, visible from dusk to midnight and showing a wide variety of detail in its cloud bands. The famous white oval designated "BA" in the south equatorial band (SEB) has overtaken the great red spot (GRS), without much of the interaction between the two features that some observers thought would happen. The split in the SEB remains prominent, and the long white diagonal rift in the NEB is growing, if anything, more prominent as time goes on. Festoons this year are mostly faint and subtle, the equatorial band is sometimes visible, and there's plenty of turbulence to see in the wake of the GRS.

Europa makes a nice transit on the 4th — it will already be in progress as dark falls, and its shadow should start to appear around dusk. By 9pm, Europa (though not its shadow) has exited the disk of Jupiter, and makes a close pass with Io, moving the other way. This is a good chance to compare the two moons when they are side by side. Io makes a similar transit on the 21st around 9-11 p.m., with the GRS rotating in to make an appearance just as Io is exiting the disk and passing close to Ganymede.

Saturn, too, is very prominent and a wonderful sight for observers. The rings remain wide open, at an inclination of 25 degrees to us (a degree less than last month, but who's counting?) and it's a perfect time to look for details in the ring system, such as gaps in the rings, and the ever elusive "spokes" which satellite images show but few earthbound observers have seen visually.

Both Saturn and Jupiter travel through star-laden fields, and often have stars visible nearby which are similar in magnitude to the planet's moons. Can you tell them apart? I often can — stars, being point sources, "twinkle" (vary in brightness or position as our turbulent atmosphere changes) much more than planetary moons do, and the color of the moons seems different, a yellowed hue that I never see in stars. It's a fun challenge, especially with Saturn — make a sketch of the objects you see near the planet, and try to guess which ones are moons and which aren't, then check your work later to see how well you did. With Jupiter, extra credit if you can tell the moons apart without looking them up first. Ganymede is easy — most good telescopes will show a disk which is clearly bigger than that of the other three Galilean moons — but the others are more difficult. See if you can tell by color differences, or by the distance from the planet.

The other three naked-eye planets are also visible in the evening sky, and by the end of the month they're all grouped fairly closely together. Mercury reaches superior conjunction on April 7, but becomes visible in the latter half of the month, the best evening apparition it will have this year. It joins its brighter sibling Venus, which lies below Mercury in the evening twilight sky, and dimmer but higher Mars, which gets quite close to Saturn by month's end. It should be fun to compare the color of the red planet to the nearby reddish star Aldebaran. Not much detail will be visible on any of these planets (though Mercury should be around half phase, moving into crescent phase at month's end) but they should make a very nice naked-eye or binocular grouping.

The outer planets — Uranus, Neptune, and Pluto — are very late evening or morning objects. Pluto, in Ophiuchus, rises a bit before midnight, while Uranus and Neptune, in Capricornus, rise a few hours later.